d) The sea of native grassland that once carpeted the United States' Great Plains has steadily been disappearing — replaced at a disconcerting rate by trees and shrubs.
a) The main tool used to drive out the trees and encourage the grasses is controlled fires, set every few years.
b) Yet these fires have not halted the advance of trees and shrubs.
e) Now, Dirac Twidwell, a range ecologist at the University of Nebraska, says he’s figured out why.
c) The problem is not that controlled burns are too destructive, he says. It’s that they’re not destructive enough.